MappingCollectionConstraintValidator.php

<?php

declare (strict_types=1);
namespace Drupal\Core\Validation\Plugin\Validation\Constraint;

use Drupal\Core\Config\Schema\Mapping;
use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\Constraints\Collection;
use Symfony\Component\Validator\Constraints\CollectionValidator;
use Symfony\Component\Validator\Exception\UnexpectedTypeException;

/**
 * Validates the MappingCollection constraint.
 */
class MappingCollectionConstraintValidator extends CollectionValidator {
  
  /**
   * {@inheritdoc}
   */
  public function validate(mixed $value, Constraint $constraint) : void {
    if (!$constraint instanceof MappingCollectionConstraint) {
      throw new UnexpectedTypeException($constraint, Collection::class);
    }
    if (NULL === $value) {
      return;
    }
    if (!$this->context
      ->getObject() instanceof Mapping) {
      throw new UnexpectedTypeException($this->context
        ->getObject(), Mapping::class);
    }
    $value = $this->context
      ->getObject()
      ->getIterator();
    parent::validate($value, $constraint);
  }

}